iOS (Jailbroken): If you've ever wanted to secretly take a picture of something with your iPhone without anybody knowing, the default camera app is not the way to do it. But if you've jailbroken your phone, you can install SlyCam, from Cydia, to take photos from your Notification Center.

Once installed and activated, you'll get a new + icon in your Notification Center. Hit that, and a photo will be taken and saved to your Photo Stream, without any further indication that anything's happened. If you do want a miniature viewfinder, you can hit the power button to the left of the + button and one will pop up. Be careful, since having a viewfinder kind of defeats the purpose of secretly taking photos with your phone, if anybody is standing behind you.